1. Field of the Invention
The present invention is directed to heat recovery power cycles. In particular, to recuperated Rankine type heat recovery power cycles and/or recuperated closed Rankine/Brayton cycles.
2. Description of the Related Technology
A recuperated Rankine Cycle, as shown in FIG. 1, is a major improvement over a non-recuperated cycle for heat source temperatures above about 250° F.-350° F. (depending on the working fluid and operating conditions). Recovering heat in the recuperator from the expander exhaust, heat that would otherwise be wasted in the condenser, and using this heat to pre-heat the working fluid entering the heat source to working fluid heat exchanger, increases the power output of the cycle and reduces the heat load on the condenser, as compared to a non-recuperated cycle. However, there is a problem with this cycle. Increasing the temperature of the working fluid entering the heat source exchanger raises the final heat source fluid exit temperature, thus reducing the amount of heat that can be transferred to the cycle.
In the last 30 years or so there have been a number of improvements to Rankine type cycles, all of these ideas aiming to extract more energy from the waste heat stream and/or to limit the amount of heat rejected in the condenser compared to the FIG. 1 cycle. Following are some examples of these improvements.
In U.S. Pat. No. 7,287,381 B1 the cold working fluid flow from the pump is split with a first part directed to the recuperator with the second part directed to a lower temperature heat source fluid heat to a working fluid heat exchanger with the flows recombining at an intermediate point between the lower temperature heat source to the working fluid exchanger and a higher temperature heat source to the working fluid exchanger. The decreased cold side flow to the recuperator caused by the flow split, allows a greater temperature rise in the working fluid. The part of the working fluid flow that goes directly from the pump to the lower temperature heat source fluid to the working fluid exchanger without recuperation results in a lower final heat source fluid exhaust temperature and the transferring of additional heat to the working fluid. The total working fluid mass flow is increased, when compared to the typical FIG. 1 cycle, thus increasing power. However, the method described is limited to cycle conditions in which there is no phase change of the working fluid in either the recuperator or the low temperature section of the waste heat fluid to working fluid heat exchanger.
U.S. Pat. No. 4,489,563 describes a Kalina cycle. This dual fluid (ammonia and water) cycle improves performance in a number of ways including capturing some of the condensing heat, reducing the approach temperature throughout the heat transfer process, and by lowering the condensing pressure. This cycle is especially efficient in the waste heat temperature ranges of 100° C. to approximately 200° C. This cycle also uses the concept of splitting the pump flow between a recuperator heated working fluid stream and a heat source heated working fluid stream but the concept is limited to a multi-component fluid working fluid with evaporation of a portion of one of the fluid components taking place in at least one heat source heat exchanger and condensation of a portion of one of the components of the working fluid talking place in at least one of the recuperators. A disadvantage of this cycle is its complexity, the somewhat corrosive nature of ammonia/water working fluid and the toxicity of ammonia.
U.S. Pat. No. 6,857,268B discloses a cascading closed loop cycle that uses the recuperated heat from a first expander to heat the working fluid to a second expander, with a portion of the remaining first expander heat combining with recuperated heat from the second expander to provide preheat for the second expander's working fluid. This is a super critical cycle with propane as the suggested preferred working fluid. The cycle is significantly more efficient than the single recuperated cycle that is shown in FIG. 1 for waste heat temperatures above about 650° F. A disadvantage of this cycle is the need for two expanders.
U.S. Pat. No. 8,474,262 discloses an advanced tandem organic Rankine cycle that combines two of the FIG. 1 cycles with the exiting heat source fluid heat from the high temperature cycle used as the incoming heat source heat to the second cycle with the intermediate heat source fluid temperature selected to optimize the performance. This is also a super critical cycle with propane as the preferred working fluid. This cycle performs best above a waste heat temperature of about 600° F. This cycle also uses two expanders which add to its complexity. The performance is a few percentage points better than the above cascading closed loop cycle for most applications.
Various binary cycles have also been proposed with many operating in geothermal applications. These cycles use the condensing heat from a higher temperature Rankine cycle as the input heat to a lower temperature cycle. While complex, these cycles are well suited for low temperature heat source applications.
Considerable work has also been done optimizing working fluid selection to best fit the particular operating conditions of waste heat and condensing temperature for the FIG. 1 cycle. An example is GE's ORegen™ Cycle which uses cyclopentane as a working fluid. This fluid extracts more power directly in the expansion compared to most other fluids and has a relatively high thermal stability limit making it well suited for recovering power from gas turbine and piston engine exhaust streams. By extracting more power directly from the expansion means a lower expander exit temperature and therefore less heat to be recycled through the recuperator thus decreasing the size and cost of this exchanger. A disadvantage of cyclopentane is that the condensing pressure is sub-atmospheric at condenser coolant temperatures under about 40° C. Thus the full thermodynamic benefits of the fluid can not be utilized at normal and low ambient temperatures without resulting in the danger of air leaking into the condenser and producing a potentially explosive mixture.
The present invention provides a simple high efficiency cycle operating at super critical conditions with the working fluid phase change from a liquid to a super critical fluid occurring in both the recuperator and a lower temperature heat source to working fluid heat exchanger, with a higher temperature heat source to working fluid exchanger operating only as a superheater adding temperature to the already super critical working fluid.